1. Field of the Invention
The present invention relates to a vehicle and a nonlinear control method for a vehicle.
2. Background Art
The operation of a vehicle can include controlling any of a number of systems within the vehicle. For example, the speed of a vehicle may be controlled by controlling the torque output of the engine or other torque producing devices. Further, a spark-ignition (SI) engine that is equipped with electronic throttle control (ETC) has three actuators capable of modifying torque independently of driver input. These modifiers are the throttle angle, the fueling rate, and the spark timing. The engine torque response to throttle angle change may be relatively slow compared to the other two methods, mainly due to the dampening effect of the intake manifold volume.
Despite the slow response, changing the throttle angle remains an effective means for controlling the torque production of the engine, because it has a wide range of authority and does not compromise the efficiency of combustion. Conversely, the torque response to changing the fueling rate and the spark timing is much faster; however, neither of these modifiers has the range of authority of changing the throttle angle. Reducing the engine torque by changing the fueling rate in an SI engine has poor resolution. In addition, changing the spark timing can result in a lower combustion efficiency which has an adverse effect on fuel economy.
The above considerations suggest that in cases where fast response is not a primary concern, the throttle angle is the most suitable lever for engine torque control. Hence, it is the most appropriate and sufficiently fast actuator in the case of vehicle speed related functions. The vehicle speed related functions include such things as a driver initiated acceleration request, a desired speed as set in a cruise control (CC) system, and a vehicle speed limit (VSL) that is a predetermined upper speed limit for vehicle operation. Recognizing that the longitudinal motion of the vehicle is heavily influenced by nonlinear factors—e.g., aerodynamic drag—it is natural to introduce nonlinearity into the control method to address this. In addition, using a nonlinear function to control a relatively slow control lever, such as an engine throttle, can increase the response of the control lever, thereby improving vehicle control.
One method of controlling the vehicle speed with a cruise control system is described in U.S. Pat. No. 5,137,104 issued to Etoh on Aug. 11, 1992. Etoh describes determining a driving force of an engine to maintain a target vehicle speed in accordance with a nonlinear relationship between the target vehicle speed and a target variable. The Etoh method uses a conversion coefficient based on the target vehicle speed, that is chosen from a lookup table. The conversion coefficient is then applied to a vehicle speed error, which is added to a throttle angle error term, which is then applied to a drive circuit to actuate a throttle valve.
Although the conversion coefficient table is based on a nonlinear relationship between vehicle speed and throttle angle, the equation used by the drive circuit to control the throttle angle is actually linear. Moreover, the nonlinear relationship used to determine the conversion coefficient is based on a target vehicle speed, not a vehicle speed error which considers the current vehicle speed. In addition, because the conversion coefficient is taken from a table, an elaborate interpolation scheme must be used when the target vehicle speed does not exactly match a table value.
Therefore, a need exists for a vehicle and a nonlinear control method for a vehicle which improves the response of one or more vehicle system controls.